ABSTRACT
This chapter provides the foundations for the formulation of mass transfer problems. It presents the governing equations for mass transfer in binary systems. Since more than one species is involved in the transport process, one has to be precise in defining concentrations, velocities, and mass/molar fluxes. Velocity of the mixture as a whole is different from individual species velocities. Heterogeneous chemical reactions take place at phase interfaces, such as catalytic surfaces. Contrary to the common belief, it is the gradient of partial molar Gibbs energy and not the concentration gradient that is responsible for mass transfer taking place. A. Fick describes his purpose as follows: “A few years ago Graham published an extensive investigation on the diffusion of salts in water, in which he more especially compared the diffusivity of different salts. In the literature, Fick’s second law of diffusion is used for diffusion in solids and stationary liquids.
